Photonic integrated circuits and systems, commonly referred as “integrated photonics”, involve fabricating both optical devices and electrical devices on a same substrate, usually a semiconductor substrate such as silicon (Si) or silicon-on-insulator (SOI) substrate. Using semiconductor fabrication techniques similar to those employed in manufacturing integrated circuits (ICs), one is able to integrate, on a same semiconductor substrate, miniaturized optical components together with electrical components, thereby performing signal processing or other circuit functions in both optical and electrical domains. Since a PIC is usually fabricated on a single substrate, it is sometimes referred as a “monolithic electro-optical (E-O) system” or a “monolithic E-O circuit”. The E-O components employed by a PIC fabricated on a substrate are thus often referred as “monolithic E-O components”.
A monolithic E-O modulator, such as a Mach-Zehnder modulator (MZM), is a PIC often employed by various digital communication systems. Miniaturized optical components (such as waveguides, optical couplers, optical splitters, phase shifters, and the like) may be integrated with miniaturized electrical components (such as resistors, capacitors, diodes, electrodes, and the like) to realize a monolithic MZM. Specifically, a monolithic MZM may include, among other components, one or more phase shifters. A phase shifter is a component that introduces a certain amount of phase shift to the optical wave passing through the phase shifter. The amount of phase shift induced therein depends on a controlling or modulating voltage (i.e., the “signal”) applied to the phase shifter.
Monolithic phase shifters, as conventionally built, face several practical challenges related to signal loss, especially for high-speed applications. That is, for a conventional phase shifter, a high-frequency modulating voltage is not able to effectively modulate the optical wave by changing its phase, resulting in a loss of the modulating signal in the optical wave. In certain scenarios, the signal loss at a phase shifter may be so significant that the phase shifter becomes inefficient, even to the point of not being able to perform its designed function. This in turn negatively affects the performance of the E-O modulator using the phase shifter.